Amyotrophic lateral sclerosis (ALS, also known as Lou Gehrig's disease) is a progressive and fatal neurodegenerative disease. A general symptom of ALS is muscle weakness and wasting triggered by denervation at neuromuscular junctions. The majority of ALS cases are sporadic, and approximately 10% are familial. Several ALS genes have been identified as their mutation can lead to familial ALS, including two genes encoding RNA processing proteins TDP-43 and fused in sarcoma/translocated in liposarcoma (FUS/TLS). FUS is a ubiquitously expressed multi-domain RNA-binding protein. In neurons and glial cells, FUS is almost exclusively localized to the nucleus but is also reported to transport mRNA for local translation in dendrites in neurons. In addition, FUS plays a role in a variety of processes including nucleocytoplasmic shuttling of mRNA, transcriptional regulation and mRNA splicing. However, little is known regarding how FUS mutations cause motor neuron degeneration and ALS, which is the focus of this study. We recently published that the C-terminus of FUS, where the ALS-causing mutations are clustered, functions as an effective nuclear localization sequence (NLS). Our newly generated data suggest that a FUS- interacting protein Gemin3 plays a critical role in the perturbations caused by FUS mutations. Gemin3 can be sequestered by ALS mutant FUS, which causes reduced Gemin3-positive nuclear structures (Gems), decreased assembly of snRNPs, and attenuated spliceosome activity. The Drosophila model we established showed motor function deficiency when FUS was over-expressed in motor neurons. Interestingly, Gemin3 was also reported to be required for larval motor function in Drosophila. Moreover, we generated FUS/Gemin3 double transgenic flies and showed that expression of Gemin3 rescued the phenotypes of FUS transgenic flies. We thus hypothesize that the ALS-related FUS mutants or WT FUS with deregulated over-expression can accumulate in cytoplasm and sequester Gemin3, which results in decreased assembly of snRNPs in cytoplasm and compromised spliceosome function in the nucleus. To test the central hypothesis, three specific aims have been designed to determine the role of FUS in ALS. Aim 1 is to understand the regulation of FUS subcellular localization by the localization sequence elements within FUS as well as by its RNA binding ability. In Aim 2, we will first determine the molecular mechanism how FUS and Gemin 3 interact. We will further characterize how FUS mutations disturb Gemin 3- mediated snRNP assembly and spliceosome activity. Aim 3 will test the molecular mechanisms defined in Aims 1 and 2 using the Drosophila model. We will first determine whether motor neuron death and neuromuscular denervation are prominent in the transgenic flies with motor neuron-specific FUS expression. FUS-mediated Gemin3 sequestering and subsequent spliceosome changes will be especially tested in flies since Gemin3 over-expression rescued the motor function deficit phenotype caused by FUS. Furthermore, the significance of FUS subcellular localization and RNA binding in producing toxicity in motor neurons will be investigated. Lastly, we will carry out RNA-Seq experiment to determine the FUS-mediated splicing alterations. This project will utilize the combination of cellular and Drosophila models to investigate the FUS- mediated ALS etiology. The findings are expected to provide critical insights into the mechanisms by which FUS mutations perturb the RNA processing pathways and ultimately lead to the disease.